With rapid development in IC design and manufacturing technology, various circuits for performing different functions can be integrated into a single integrated circuit (IC) chip. Since the working voltages for respective circuits may be different, a voltage regulator circuit is employed for regulating the voltage supplied from the external voltage source to required levels. Referring to FIG. 1, a conventional voltage regulator comprises two voltage regulating units 12 and 13 for regulating the constant voltage V0 received from the external voltage source 11 into two voltages V1 and V2 to be respectively provided for two different circuits (not shown) of the IC chip 10. In each of the voltage regulating units 12 and 13, the emitter of the bipolar junction transistor 120 (130) is connected to the external voltage source 11, and the base and collector of the bipolar junction transistor 120 (130) are electrically connected to the IC chip 10 via a control pin 100 (102) and a power input pin 101 (103), respectively. The operation principle of the voltage regulator circuit will be illustrated hereinafter by taking the voltage regulating unit 12 as an example.
The constant voltage V0 provided by the external voltage source 11 is transmitted to the emitter of the bipolar junction transistor 120 to be regulated according to the current intensity flowing through the control pin 100. The current intensity is controlled by the power control means of the IC chip 10. Then, a regulated voltage V1 is outputted from the collector of the bipolar junction transistor 120 and provided for a first circuit portion of the IC chip 10 via the buffer capacitor 121 and the power input pin 101. Likewise, a regulated voltage V2 can be provided for a second circuit portion of the IC chip 10 via the buffer capacitor 131 and the power input pin 103 after the regulated operation of the constant voltage V0 by the bipolar junction transistor 130.
The above voltage regulator circuit is advantageous to effectively provide required working voltages for the IC chip. However, there are still some shortcomings occurring in the conventional voltage regulator circuit. For example, when the voltage V0 supplied from the external voltage source 11 is about 3˜5 volts and the voltage V1 or V2 required by the IC chip 10 is about 1˜3 volts, the current intensity flowing through the control pin 100 or 102 would be in the order of tens to hundreds of milliamperes (mA). For dealing with this order of current intensity, the control pin 100 or 102 has to occupy a number of pads, and thus large area of the IC chip is wastefully occupied. In addition, once the collector of either of the bipolar junction transistors 120 or 130 is undesirably short-circuited, a large amount of current will flow through the bipolar junction transistor and may burn down the bipolar junction transistor.